“Principles have a way of enduring, as do the few irreducible individuals who maintain allegiance to them.”

Source: Aspects of the Novel (1927), Chapter Nine: Conclusion
Context: If human nature does alter it will be because individuals manage to look at themselves in a new way. Here and there people — a very few people, but a few novelists are among them — are trying to do this. Every institution and vested interest is against such a search: organized religion, the state, the family in its economic aspect, have nothing to gain, and it is only when outward prohibitions weaken that it can proceed: history conditions it to that extent.

Still Me (1999); also quoted at the Christopher Reeve Foundation http://www.christopherreeve.org/site/c.geIMLPOpGjF/b.1097025/k.6FF5/Christopher_and_Dana_Reeve.htm
Context: When the first Superman movie came out, I gave dozens of interviews to promote it. The most frequent question was: What is a hero? My answer was that a hero is someone who commits a courageous action without considering the consequences. Now my definition is completely different. I think a hero is an ordinary individual who finds the strength to persevere and endure in spite of overwhelming obstacles. They are the real heroes, and so are the families and friends who have stood by them.

Source: Thinking and Destiny (1946), Ch. 11, The Great Way, p. 699

Source: In the Name of Identity: Violence and the Need to Belong

21 : Setsudo - teaching the way of the universe
Ki Sayings (2003)

Out of Control: The New Biology of Machines, Social Systems and the Economic World (1995), New Rules for the New Economy: 10 Radical Strategies for a Connected World (1999)

"On the Method of Theoretical Physics" The Herbert Spencer Lecture, delivered at Oxford (10 June 1933); also published in Philosophy of Science, Vol. 1, No. 2 (April 1934), pp. 163-169., p. 165. [thanks to Dr. Techie @ www.wordorigins.org and JSTOR]
There is a quote attributed to Einstein that may have arisen as a paraphrase of the above quote, commonly given as “Everything should be made as simple as possible, but no simpler,” "Everything should be made as simple as possible, but not simpler", or “Make things as simple as possible, but not simpler.” See this article from the Quote Investigator http://quoteinvestigator.com/2011/05/13/einstein-simple/ for a discussion of where these later variants may have arisen.
The original quote is very similar to Occam's razor, which advocates that among all hypotheses compatible with all available observations, the simplest hypothesis is the most plausible one.
The aphorism "everything should be made as simple as possible, but no simpler" is normally taken to be a warning against too much simplicity and emphasizes that one cannot simplify things to a point where the hypothesis is no more compatible with all observations. The aphorism does not contradict or extend Occam's razor, but rather stresses that both elements of the razor, simplicity and compatibility with the observations, are essential.
The earliest known appearance of Einstein's razor is an essay by Roger Sessions in the New York Times (8 January 1950) http://select.nytimes.com/gst/abstract.html?res=F30615FE3559137A93CAA9178AD85F448585F9, where Sessions appears to be paraphrasing Einstein: “I also remember a remark of Albert Einstein, which certainly applies to music. He said, in effect, that everything should be as simple as it can be, but not simpler.”
Another early appearance, from Time magazine (14 December 1962) http://www.time.com/time/magazine/article/0,9171,872923,00.html: “We try to keep in mind a saying attributed to Einstein—that everything must be made as simple as possible, but not one bit simpler.”
1930s

Source: Nobody Knows My Name

Injunctions of Marx
Specters of Marx (1993)